1. Field of the Invention
The present invention relates to a memory device, and more particularly to a non-volatile memory and a method of manufacturing a floating gate.
2. Description of the Related Art
Electronically erasable and programmable read only memory (EEPROM), among various of non-volatile memory devices, is suitable of performing multiple operations of writing, read and erasure, and is non-volatile even when power is off. As a result, EEPROM becomes a type of memory device widely used in personal computers and other electronic equipments.
In a EEPROM, doped polysilicon is conventionally used for fabricating a floating gate and a control gate. A dielectric layer is used to separate the floating gate from the control gate, while a tunneling layer is used to separate the floating gate from the substrate. When an operation of data writing/erasing is performed on the EEPROM, a bias voltage is applied on the control gate, the source region and the drain region so as to inject charges into the floating gate or withdraw charges from the floating gate. When data is read from the memory, an operating voltage is applied on the floating gate, and because the threshold voltage of the floating gate has been changed in the earlier write/erase operation, the difference of the threshold voltages can be used for differentiating the data value of 0 and 1.
Because the floating gate is made of a semiconductor material (e.g., polysilicon), electrons injected in the floating gate will be uniformly distributed in the entire layer of the floating gate. Upon multiple operations, if defects exist in the tunneling oxide layer below the layer of the polysilicon floating gate, electric leakage may easily occur to affect the device reliability. As a result, the thickness of the tunneling oxide layer cannot be reduced and a desirable lower operating voltage is hard to be obtained.